Methods for determining the seal integrity, or seal tightness, or hermeticity of a test article, wherein a closed space is formed in the testing zone that is formed by the outer surface of the article and a transparent electrode, has been disclosed. In this method, the test article and the electrode are subjected to a difference of potentials that corresponds to the threshold value of gas discharge formation, and the inner surface of the article. The article is bathed by a test gas that diffuses through the fault into the closed space causes the formation of a discharge electron avalanche. A recording device, for example a photo camera, is positioned on the side of the transparent electrode to record the discharge. The information carrier in the known method is the photo image of the discharge electron avalanche whose shape allows evaluation of the spatial inhomogeneity of the gas emission through the surface of the article and to determine the defect, leak, or flaw size and location.
A limitation of the known method is the necessity to bathe the test article in a test gas. This complicates the testing process and excludes its application for the articles with interior volume that is filled with a working medium featuring specified properties.
There are also other known methods of nondestructive testing that do not require any test gas usage and that allow determination or detection of article faults on the basis of gas discharge visualization. According to these methods, the article being tested and the recorder, in the form of a liquid crystal cell, are placed between the electrodes, while a high voltage is applied to the electrodes, and the article is subjected to an applied electric field with field lines (vector) being normal to the article surface. The test results are indicated by the image of the gas discharge process recorded by the registration unit.
The presence of a fault is indicated by a change in the form of the gas discharge image caused by a distortion of the normal component of the electric field within the fault area. To improve the indicating potential of the test, the air gap between the article and the recording unit is maintained within the 50-100 μm range.
The character of the gas discharge image is a function of the thickness and dielectric permeability of the layers that confine the gas discharge gap, while the shape is determined by the shape of the article and is comprised of separate film exposure spots whose quantity per surface area unit is determined by the amplitude and number of pulses during the exposure to a high voltage electric field. The film exposure is caused by the avalanche-like discharge processes that linearly propagate along the electric field lines of force from the separate points of the object to the image recording unit. The discharges appear at random points of the object uniformly over the whole surface area of the gap. The presence of faults on the surface or in the volume of the article results in a distortion of the electric field symmetry in the gas discharge gap. This is reflected in the shape of the gas discharge image.
In such case, an increase in the electric field strength causes a preferential and most intensive development of avalanche-like discharges in this field, while its decrease leads to discharge weakening. Therefore the defective area will be exposed on the recording unit either more lightly or more strongly in comparison to the background, that is, to the defect-free areas. The testing sensitivity is a function of the electric field strength in the defective and defect-free areas.
A common limitation of the conventional methods is their low sensitivity in testing of articles that have outer electrically conducting layers that drastically reduce the indicating potential of the testing according to the value of the change in the normal electric field component within the defective area of the article.
There is a known method of recording air discontinuities in a solid body wherein an article being tested is placed between the electrodes to which high voltage is applied, while a glow is caused in the discharge gap, the character of the glow reflecting the inner structure of the article. The discharge glow is recorded on the photo carrier.
To determine the size of the air space in the direction of the electric field lines of force the discharge photography is carried out in two stages. While a picture is taken at the threshold of the discharge ignition, the value of the applied voltage is recorded, whereupon the voltage on the electrodes is continually increased up to a value at which a discharge occurs in the air volume of the defect that is reflected on the film. In this case the defect image will be brighter than in defect free areas.
By recording the value of the applied voltage and comparing it with the voltage at which the first picture was obtained, information is gained on the presence of defects and their location in the direction of the electric field lines of force. A disadvantage of this known method is the high error in determining the voltage values of the discharge ignition in the fault space. This substantially lowers the sensitivity of the test.
There is also a method of nondestructive testing for faulty adhesion in thin layer metal-dielectric arrangements in which the metallic substrate of the article is used as one of the electrodes. A photographic film or photo carrier is laid over the dielectric layer with the emulsion layer facing the article surface. The whole system is pressed with a roller that is rolled along the article surface, while voltage is supplied from a high voltage impulse generator to the roller functioning as a positive electrode.
After exposure the photo material is processed using a conventional photographic printing method. In such case an appearance of a fault that is of an air interlayer between the metallic substrate and the dielectric coat of the article leads to a reduction in the field strength on the outer surface of the article and, correspondingly, to weakening of the discharge that appears in the faulty area. These areas will be brighter in comparison to the defect-free areas. The sensitivity of the known testing method is determined by the character of the distribution of the normal electric field component above the surface of the article being tested and by the relationship between the faulty adhesion value and the dielectric coat thickness.
A disadvantage of this method is the requirement that a gas discharge gap be formed on the side of the dielectric layer of the article, thus precluding the possibility of its usage for articles with a metallized layers. The sensitivity of this method is low because only the normal component of the electric field will operate within the non-adhesion area.